Authentication of quantum dot security inks

ABSTRACT

A method is provided for verifying the authenticity of an article which bears a security mark. The method includes irradiating the security mark with a time-varying light source, ascertaining at least one portion of the emissions spectrum of the irradiated security mark with at least one photodetector, determining the photoluminescence lifetime of the security mark by monitoring the time or frequency response of the photodetector, and verifying the authenticity of the article only if the security mark exhibits a photoluminescence which has a lifetime that falls within the range of appropriate values for each portion of the photoluminescence spectrum for which the photoluminescence lifetime of said security mark was ascertained.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims priority to, U.S. Ser.No. 15/943,723, entitled “Methods Of Authenticating Security Inks”,having the same inventor, which was filed on Apr. 3, 2018, and which isincorporated herein by reference in its entirety; which is acontinuation of, and claims priority to, U.S. Ser. No. 14/860,681,entitled “Methods Of Authenticating Security Inks”, having the sameinventor, which was filed on Sep. 21, 2015, which issued as U.S. Pat.No. 9,964,488 on May 8, 2018, and which is incorporated herein byreference in its entirety; which is a national stage filing of, andclaims priority to, PCT/U.S. Ser. No. 16/37122, entitled “Quantum DotSecurity Inks”, having the same inventor, which was filed on Jun. 12,2016, and which is incorporated herein by reference in its entirety;which claims priority to U.S. Ser. No. 14/860,676, entitled “Quantum DotSecurity Inks”, having the same inventor, which was filed on Sep. 21,2015, which issued as U.S. Pat. No. 9,382,432 on Jul. 5, 2016, and whichis incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to security ink compositions,and more specifically to methods for using security ink compositionscontaining photoluminescent materials such as quantum dots foranti-counterfeit or authentication purposes, and to methods for uniquelyidentifying the presence of photoluminescent materials by spectrallyresolving their photoluminescence lifetime.

BACKGROUND OF THE DISCLOSURE

Watermarks have been integrated into documents to verify authenticitysince at least as early as the 1200's. The concept was to apply aunique, hard-to-replicate design feature that could quickly beidentified by a stakeholder. This type of approach was applied in U.S.Pat. No. 353,666 (Crane, Jr.), entitled “Watermarked Paper” and filed in1886, which notes that “when the paper thus produced is examined againstthe light”, unique features can be observed.

Photoluminescence (PL) is the emission of light (electromagneticradiation, photons) after the absorption of light. It is one form ofluminescence (light emission) and is initiated by photoexcitation(excitation by photons). Following photon excitation, various chargerelaxation processes can occur in which other photons with a lowerenergy are re-radiated on some time scale. The energy difference betweenthe absorbed photons and the emitted photons, also known as Stokesshift, can vary widely across materials from nearly zero to 1 eV ormore. Time periods between absorption and emission may also vary, andmay range from the short femtosecond-regime (for emissions involvingfree-carrier plasma in inorganic semiconductors) up to milliseconds (forphosphorescent processes in molecular systems). Under specialcircumstances, delay of emission may even span to minutes or hours.Further, for a given material or mixture of materials, the emissionlifetime can depend on the excitation and emission wavelength.

Some uses of luminescent security inks for authentication are known tothe art. This may be appreciated, for example, with respect to U.S. Pat.No. 2,742,631 (Rajchman et al.), entitled “Methods For Recording AndTransmitting Information Using Phosphors”, which was filed in 1954, andU.S. Pat. No. 3,614,430 (Berler), entitled “Fluorescent-Ink-InprintedCoded Document And Method And Apparats For Use In Connection Therewith”,which was filed in 1969.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a typical overt authenticationsystem, wherein an observer excites the security ink with a light sourceand visually observes the resulting visible fluorescence.

FIG. 2 is a schematic illustration of covert authentication system inwhich the security ink is excited with a time-varying light source (suchas, for example, a blue or UV LED), and wherein the time-varyingfluorescence is subsequently measured by a photodetector. In this case,the spectral resolution is achieved intrinsically by the choice of thephotodetector.

FIG. 3 is a schematic illustration of covert authentication system inwhich the security ink is excited with a time-varying light source (suchas, for example, a blue or UV LED), and wherein the time-varyingfluorescence is subsequently measured by a photodetector after passingthrough a spectrum selecting component. In this case, the spectralresolution is achieved by choice of the spectrum selecting component (insome embodiments, the spectrum selecting component may be a filter orfilm of quantum dots) and also by choice of photodetector.

FIG. 4 is a schematic illustration of a covert authentication system inwhich the security ink is excited with a time-varying light source (suchas, for example, a blue or UV LED), and wherein the time-varyingfluorescence is subsequently measured by multiple photodetectors afterpassing through one or more spectrum selecting components. In this case,the spectral resolution is achieved by choice of the spectrum selectingcomponents in front of each detector (in some embodiments, thesespectrum selecting components may be different filters or films ofdifferent quantum dots), and also by the choice of the photodetectors.

FIG. 5 is a graph of a typical absorption and photoluminescence spectrafor exemplary CuInZnSeS quantum dots. These QDs are substantially freeof toxic elements and are believed to be non-carcinogenic. The QDs havean emission quantum yield of >70%.

FIG. 6 is a graph of the photoluminescence arising from two differentmixtures of CuInZnSeS quantum dots inks on a paper substrate. The shapeof the spectrum, including the number of peaks, number of troughs, slopeof the spectrum, and other signatures, is tailored based on the size andcomposition of the quantum dots chosen. The QDs mixtures have anemission quantum yield of >50%.

FIG. 7 is a graph of the photoluminescence decay arising from twodifferent photoluminescent materials that may be used in security inks.The PL decay from different CuInZnSeS QDs having single exponentialdecays of 417 ns (CIS QD 1, squares) and 209 ns (CIS QD 2, circles) iscompared with typical CdSe QDs having a 30 ns lifetime (up triangles)and rhodamine 6G dye having a 5 ns lifetime (down triangles).

FIG. 8 is a flowchart illustrating a first embodiment of the methodologydisclosed herein, and in which one or more lifetimes may be measured foran emission spectrum.

FIG. 9 is a flowchart illustrating a second embodiment of themethodology disclosed herein, and in which one or more lifetimes may bemeasured for an emission spectrum by measuring the phase differencebetween a signal A which is used to produce the time-varying light usedto irradiate a mark, and a signal B produced by a photodetector used todetect emissions from the irradiated mark.

SUMMARY OF THE DISCLOSURE

In one aspect, a security ink is provided which comprises (a) a liquidmedium; and (b) a plurality of quantum dots disposed in said mediumwhich, upon excitation with a light source, exhibit a quantum yieldgreater than 30%, and a photoluminescence which has at least onelifetime of more than 40 nanoseconds but less than 1 millisecond andwhich varies by at least 5% across the emission spectrum of the quantumdots.

In another aspect, and in combination with a security ink, an opticalapparatus for analyzing said security ink is provided. The opticalapparatus comprises (a) a time-varying light source which excites saidsecurity ink, thereby causing said security ink to emit an emissionspectrum having first and second distinct regions which arecharacterized by first and second distinct lifetimes; (b) at least onephotodetector; (c) a first optical element which allows only said firstregion of said emission spectrum from an optical signal to pass throughit; (d) a second optical element which allows only said second region ofsaid emission spectrum from an optical signal to pass through it; and(e) an electronics module which determines the photoluminescencelifetime of said security ink over said first and second regions bymonitoring at least one time or frequency response of said at least onephotodetector.

In a further aspect, a method is provided for verifying the authenticityof an article which bears a security mark. The method comprises (a)irradiating the security mark with a time-varying light source; (b)ascertaining at least one portion of the emissions spectrum of theirradiated security mark with at least one photodetector; (c)determining the photoluminescence lifetime of said security mark bymonitoring the time or frequency response of said photodetector; and (d)verifying the authenticity of the article only if the security markexhibits a photoluminescence which has a lifetime that falls within therange of appropriate values for each portion of the photoluminescencespectrum for which the photoluminescence lifetime of said security markwas ascertained.

In still another aspect, method for authenticating an article asbelonging to a set of authentic articles, wherein each member of the setof authentic articles bears an inked security mark that emits aphotoluminescence spectrum in response to being excited by atime-varying light source that emits pulses of light at a plurality offrequencies, and wherein the emitted photoluminescence spectrum ischaracterized by a range of lifetimes. The method comprises (a)determining whether the article to be authenticated contains a securitymark; (b) if the article contains a security mark, irradiating thesecurity mark with an instance of said time-varying light source, anddetermining the upper and lower bounds for the lifetime of at least oneportion of said photoluminescence spectrum emitted by the irradiatedsecurity mark; and (c) authenticating the article only if (i) thearticle contains a security mark, and (ii) the irradiated security markemits a photoluminescence having a lifetime that falls withinpredetermined upper and lower bounds characteristic of an authenticarticle for each portion of the photoluminescence spectrum for which thephotoluminescence lifetime of said security mark was ascertained.

In yet another aspect, a method is provided for authenticating anarticle as belonging to a set of authentic articles, wherein each memberof the set of authentic articles bears an inked security mark that emitsa photoluminescence spectrum in response to being excited by atime-varying light source, and wherein the emitted photoluminescencespectrum is characterized by a range of lifetimes. The method comprises(a) determining whether the article to be authenticated contains asecurity mark; (b) if the article contains a security mark, (i)irradiating the security mark with a time-varying light source, whereinsaid time-varying light is created with a first electrical signal, and(ii) capturing a portion of the emission spectrum of the irradiatedarticle with at least one photodetector; (c) determining the phasedifferences between the first electrical signal and a second electricalsignal of the same frequency which is received from said photodetectorin response to the captured portion of the emission spectrum; (d)determining the lifetimes of the photoluminescence of the irradiatedsecurity mark from the determined phase differences; and (e)authenticating the article only if (i) the article contains a securitymark, and (ii) the irradiated security mark emits a photoluminescencespectrum whose determined lifetimes fall within the range of lifetimescharacteristic of an authentic article.

DETAILED DESCRIPTION 1. Background

Colloidal semiconductor nanocrystals, commonly known as quantum dots(QDs), provide various size-tunable optical properties, including PL,and may be inexpensively processed from liquids. In particular, they arevery effective at absorbing a broad spectrum of light and thenconverting that energy into emitted light of a single color that isdetermined by their size. Optical properties (such as, for example,absorption and emission spectra, PL lifetimes and Stokes shift) can beprogrammed into these materials by tailoring the manufacturingconditions to realize different sizes, shapes, compositions, and/orheterostructuring. This fundamental property of QDs has spurred researchand development of fluorescence biolabeling, color-specificlight-emitting-diodes, and vibrant displays. However, the currentgeneration of QDs are toxic and far too expensive to reach most markets.There is thus a unique opportunity for QDs that are both low-cost andnon-toxic as active elements of luminescent composites for security inks(e.g., overt and covert optical features) and other applications (e.g.,lighting, solar, safety, design).

It became clear in the late 1990's that the emerging technology of QDsmight be particularly well suited as fluorophores for security inks. Oneof the earliest reports of QD security inks may be found in U.S. Pat.No. 6,576,155 (Barbara-Guillem), entitled “Fluorescent Ink CompositionsComprising Functionalized Fluorescent Nanocrystals”, which was filed in1998. This reference notes that a “mark is invisible to the unaided eye,but that can be detected as fluorescence upon excitation with anactivating light of a suitable excitation wavelength spectrum.”

The concept of using the fluorescence lifetime of quantum dots may befound in U.S. Pat. No. 6,692,031 (McGrew), entitled “Quantum DotSecurity Device And Method”, which was filed on Sep. 18, 2001. McGrewsaw QDs as being advantageous over dyes (alternative fluorophore)because dyes typically have a very fast PL lifetime, on the order of afew nanoseconds. However, McGrew incorrectly claimed that the lifetimeof typical CdSe QDs was “hundreds of nanoseconds”, which is only thecase if the QDs are very poorly passivated such that the emission arisesfrom surface states. In that case, the PL QY of the dots is very low,typically <1%, with the result that the emission is far too weak to beof practical use. However, in well passivated CdSe-based QDs that havehigh QY (>50%), the emission lifetime is much faster, on the order of15-30 ns at room temperature (see, e.g., Li, L. A.; Pandey, A.; Werder,D. J.; Khanal, B. P.; Pietryga, J. M.; Klimov, V. I. J. Am. Chem. Soc.2011, 133, 1176). Similarly, typical high-efficiency inorganic phosphorssuch as yttrium aluminum garnet (YAG) have PL lifetimes on the order of20 ns (see, e.g., Allison, S. W.; Gillies, G. T.; Rondinone, A. J.;Cates, M. R. Nanotechnology 2013, 14, 859).

In more recent references such as U.S. 2009/0045360 (Wosnick), entitled“Quantum Dot-Based Luminescent Marking Material”, and U.S. 2008/0277626(Yang), entitled “Quantum Dot Fluorescent Inks”, the focus has been onthe spectral signatures of a QD based security ink. For example, Wosnickteaches “materials comprising two or more luminescent marking materials,wherein each luminescent marking material, when exposed to activatingradiation, has a unique narrow emission band”. Yang teaches materialswith a wider range of emissions between “about 450 nm and 2500 nm”. Yangalso teaches semiconductors such as CuInGaS₂, CuInGaSe₂, AgInS₂,AgInSe₂, and AgGaTe₂ as examples of materials that the “quantum dot corecan comprise”.

Nanocrystal quantum dots of the I-III-VI class of semiconductors, suchas CuInS₂, are of growing interest for applications in optoelectronicdevices such as solar photovoltaics (see, e.g., PVs, Stolle, C. J.;Harvey, T. B.; Korgel, B. A. Curr. Opin. Chem. Eng. 2013, 2, 160) andlight-emitting diodes (see, e.g., Tan, Z.; Zhang, Y.; Xie, C.; Su, H.;Liu, J.; Zhang, C.; Dellas, N.; Mohney, S. E.; Wang, Y.; Wang, J.; Xu,J. Advanced Materials 2011, 23, 3553). These quantum dots exhibit strongoptical absorption and stable efficient photoluminescence that can betuned from the visible to the near-infrared (see, e.g., Zhong, H.; Bai,Z.; Zou, B. J. Phys. Chem. Lett. 2012, 3, 3167) through composition andquantum size effects. In fact, Gratzel cells sensitized by specificallyengineered quantum dots have recently been shown to offer excellentstability and certified power conversion efficiencies of >5%. (seeMcDaniel, H.; Fuke, N.; Makarov, N. S.; Pietryga, J. M.; Klimov, V. I.Nat. Commun. 2013, 4, 2887). Alloyed CuInZnSeS QDs are particularlyattractive for luminescent security inks because of their low toxicity,long term stability, nearly ideal PL lifetime, and other unique opticalproperties. In the security inks and methods of authentication disclosedin that reference, spectrally resolving the PL lifetime is surprisinglysimple and cost-effective using this material.

The current generation of security inks and methods of theirauthentication have several major drawbacks that limit their utility.First, optical spectra alone can be easily reproduced by one or acombination of fluorophores that are widely available. Second, althoughone simple way to distinguish between such fluorophores could beachieved by resolving their PL lifetime, the PL lifetime of mostemissive materials is less than 30 nanoseconds or longer than 100's ofmicroseconds. Distinguishing between a PL lifetime of a few nanoseconds(or less) and tens of nanoseconds is a non-trivial undertaking withtypical electronics, since it requires pulsed excitation and detectionwith bandwidths on the order of hundreds of 1 MHz.

For example, at present, an off the shelf LED which may be obtained fromtypical suppliers at a cost of a few dollars has a rise and fall time ofabout 10 ns, or a 20 ns pulse width (at shortest). Upgrading to a 1 nspulse width LED will cost about $3,000 retail, while the price of a 200ps pulse-capable LED is about $10,000. In order to accurately measurethe PL lifetime of a material, the excitation pulse width should beshorter than the PL lifetime, since otherwise the measurement willconsistently produce the LED temporal behavior only. Therefore,lifetimes longer than tens of nanoseconds are needed in order todistinguish materials inexpensively, since otherwise, costlyfast/frequent pulses and ultrafast detection are required.

Conversely, lifetimes which are too long-for example, manganese-dopedzinc sulfide nanocrystals have a 2 ms PL lifetime (see He, Y.; Wang,H.-F.; Yan, X.-P. Anal. Chem. 2008, 80, 3832)-will take too long forauthentication, since in that case, the excitation frequency must bemuch less than the inverse of the PL lifetime. For example, if oneattempted to pulse a 2 ms fluorophore at 50 kHz, the signal would not beable to decay appreciably between pulses (½ ms=0.5 kHz<<50 kHz). Inorder to build up signal to noise, it is estimated that at least 1000cycles must be completed. Hence, a 2 ms PL lifetime needs at least 2seconds worth of data for each frequency, while a 500 ns PL lifetimewould need only about 0.5 ms for each frequency.

Thirdly, most QD materials available today are highly hazardous. The useof cadmium-based fluorophore is a non-starter for most security inkapplications, since it is a known carcinogen that bio-accumulates in thehuman body. The most common cadmium-free QD material, indium phosphide,is also a known carcinogen. For near-IR emission, lead-based QDs aretypically utilized. There is a clear and urgent need for QD fluorophoreswhich are non-carcinogenic and have PL lifetimes of order 100's ofnanoseconds.

In addition, there are also problems with methods of authentication, inpart because the security ink technology was not conceived whichdemanded new authentication concepts. Although McGrew teaches that PLlifetimes can be combined with spectral signatures for enhancedauthentication, the reference does not teach spectrally resolving the PLlifetime. A material which contains a PL lifetime that varies over thedetection spectral bandwidth would produce an average lifetime ifmeasured over the entire spectrum. Such an average would not be a singleexponential decay, but rather a multi-exponential linear sum of thecontributing decays. A single exponential decay is important forlow-cost authentication because it allows for simple, unambiguousdetermination of the lifetime. Further, typical methods for spectrallyresolving a lifetime would require the pulsed emission to pass through adiffraction grating or prism in order to split the spectrum spatiallyfor detection. Such spectral splitting requires large volumes and, insome cases, moving parts, which slows the authentication process and/orincreases the size of the authenticator. Hence, in order to takeadvantage of the security inks disclosed herein, new methods of compactand rapid authentication are needed wherein the PL lifetime isspectrally resolved (or, equivalently, wherein the PL spectrum istemporally resolved).

2. Overview

Full spectrum (visible to near-IR, 400-1400 nm) photoluminescentnon-toxic security inks are needed to create unique spectral andtemporal signatures on high value items including, but not limited tobanknotes, credit cards, important documents, pharmaceuticals, andluxury goods. Existing methods for rapid, compact, and low-costauthentication of these security inks have not yet been envisioned, butare required in parallel.

Novel security inks are disclosed herein which, in a preferredembodiment, contain non-carcinogenic QDs having tunable PL spectra withpeaks in the visible (400-650 nm) to near-IR (650-1400 nm) andspectrally varying PL lifetimes in the optimal range of 100-1000 ns. Insome embodiments, the ink may contain multiple sizes and/or compositionsof QD emitters to modify the spectrum and/or temporal characteristicsfurther. A preferred, though non-limiting, photoluminescent material forthis purpose is CuInZnSeS QDs.

Methods of authentication of these security inks are also disclosedwhich involve pulsed LED excitation and spectrally-resolved detection.The PL decay may be characterized in the frequency domain or in the timedomain by probing of the delay between detected photons and theexcitation. This may be accomplished, for example, by measuring thephase relationship between the excitation waveform and the detectedwaveform. The spectral resolving capability may be achieved by filteringthe light prior to detection with a long pass, short pass, or band passfilter. An exemplary long-pass filter material for this purpose maycomprise the same or similar QDs as are used in the ink; however, theQDs in the filter material are preferably rendered non-emissive orweakly-emissive.

The compositions, systems and methodologies disclosed herein representan improvement over previous generations of authentication technologiesin which it was typical for only the spectral signatures to be observed,since temporal characterization was not economically viable. Moreover,in previous authentication methodologies, the temporal response of asecurity ink was not spectrally resolved. The compositions, systems andmethodologies disclosed herein may be utilized to provide a simple,safe, rapid, and cost-effective solution to the counterfeiting of highvalue items.

3. Definitions and Abbreviations

The following explanations of terms and abbreviations are provided tobetter describe the present disclosure and to guide those of ordinaryskill in the art in the practice of the present disclosure. As usedherein, “comprising” means “including” and the singular forms “a” or“an” or “the” include plural references unless the context clearlyindicates otherwise. The term “or” refers to a single element of statedalternative elements or a combination of two or more elements, unlessthe context clearly indicates otherwise.

Unless explained otherwise, all technical and scientific terms usedherein have the same meaning as commonly understood to one of ordinaryskill in the art to which this disclosure relates. Suitable methods andcompositions are described herein for the practice or testing of thecompositions, systems and methodologies described herein. However, it isto be understood that other methods and materials similar or equivalentto those described herein may be used in the practice or testing ofthese compositions, systems and methodologies. Consequently, thecompositions, materials, methods, and examples disclosed herein areillustrative only, and are not intended to be limiting. Other featuresof the disclosure will be apparent to those skilled in the art from thefollowing detailed description and the appended claims.

Unless otherwise indicated, all numbers expressing quantities ofcomponents, percentages, temperatures, times, and so forth, as used inthe specification or claims are to be understood as being modified bythe term “about.” Unless otherwise indicated, non-numerical propertiessuch as colloidal, continuous, crystalline, and so forth as used in thespecification or claims are to be understood as being modified by theterm “substantially,” meaning to a great extent or degree. Accordingly,unless otherwise indicated implicitly or explicitly, the numericalparameters and/or non-numerical properties set forth are approximationsthat may depend on the desired properties sought, the limits ofdetection under standard test conditions or methods, the limitations ofthe processing methods, and/or the nature of the parameter or property.When directly and explicitly distinguishing embodiments from discussedprior art, the embodiment numbers are not approximates unless the word“about” is recited.

Carcinogen: A material that has been shown to directly or indirectlycause cancer in any mammal.

Phase Measurement Device: A device that measures phase. Examplesinclude, but are not limited to, lock-in amplifiers, impedance gainphase analyzers, oscilloscopes, and network analyzers.

Photoluminescence (PL): The emission of light (electromagneticradiation, photons) after the absorption of light. It is one form ofluminescence (light emission) and is initiated by photoexcitation(excitation by photons).

Polymer: A large molecule, or macromolecule, composed of many repeatedsubunits. Polymers range from familiar synthetic plastics such aspolystyrene or poly(methyl methacrylate) (PMMA), to natural biopolymerssuch as DNA and proteins that are fundamental to biological structureand function. Polymers, both natural and synthetic, are created viapolymerization of many small molecules, known as monomers. Exemplarypolymers include poly(methyl methacrylate) (PMMA), polystyrene,silicones, epoxy resins, and nail polish.

Toxic: Denotes a material that can damage living organisms due to thepresence of phosphorus or heavy metals such as cadmium, lead, ormercury.

Quantum Dot (QD): A nanoscale particle that exhibits size-dependentelectronic and optical properties due to quantum confinement. Thequantum dots disclosed herein preferably have at least one dimensionless than about 50 nanometers. The disclosed quantum dots may becolloidal quantum dots, i.e., quantum dots that may remain in suspensionwhen dispersed in a liquid medium. Some of the quantum dots which may beutilized in the compositions, systems and methodologies described hereinare made from a binary semiconductor material having a formula MX, whereM is a metal and X typically is selected from sulfur, selenium,tellurium, nitrogen, phosphorus, arsenic, antimony or mixtures thereof.Exemplary binary quantum dots which may be utilized in the compositions,systems and methodologies described herein include CdS, CdSe, CdTe, PbS,PbSe, PbTe, ZnS, ZnSe, ZnTe, InP, InAs, Cu₂S, and In₂S₃. Other quantumdots which may be utilized in the compositions, systems andmethodologies described herein are ternary, quaternary, and/or alloyedquantum dots including, but not limited to, ZnSSe, ZnSeTe, ZnSTe, CdSSe,CdSeTe, HgSSe, HgSeTe, HgSTe, ZnCdS, ZnCdSe, ZnCdTe, ZnHgS, ZnHgSe,ZnHgTe, CdHgS, CdHgSe, CdHgTe, ZnCdSSe, ZnHgSSe, ZnCdSeTe, ZnHgSeTe,CdHgSSe, CdHgSeTe, CuInS₂, CuInSe₂, CuInGaSe₂, CuInZnS₂, CuZnSnSe₂,CuIn(Se,S)₂, CuInZn(Se,S)₂, and AgIn(Se,S)₂ quantum dots, although theuse of non-toxic quantum dots is preferred. Embodiments of the disclosedquantum dots may be of a single material, or may comprise an inner coreand an outer shell (e.g., a thin outer shell/layer formed by anysuitable method, such as cation exchange). The quantum dots may furtherinclude a plurality of ligands bound to the quantum dot surface.

Security ink: A liquid solution applied by inkjet printing, stamping,scribing, spraying, or other marking methods that imparts uniquelyidentifiable features onto a substrate for the purposes ofauthentication or counterfeit prevention.

Emission spectrum: Those portions of the electromagnetic spectrum overwhich a mark exhibits PL (in response to excitation by a light source)whose amplitude is at least 1% of the peak PL emission.

4. Best Mode

The preferred embodiment of the systems and methodologies disclosedherein includes the use of a security ink comprising a mixture of one ormore sizes and/or compositions of CuInZnSeS QDs (see FIGS. 5-6), and thespectrally-resolved detection of the temporal signatures (see FIG. 7) ofthe security ink with one or more photodetectors (see FIGS. 2-4). FIG. 4depicts the mode with the strongest authentication, wherein light source1 (which may be, for example, a blue or UV LED) emits a time-varyingexcitation 2 upon a security ink containing QDs 3 applied to a substrate4. Then, the time-varying photoluminescence from the ink 5 is measuredby first and second photodetectors 6 and 8 after being spectrallyresolved using first and second optical elements 7 and 9 which may be,for example, optical filters. In some embodiments, the first and secondoptical elements 7 and 9 may comprise thin films containing non-emissiveversions of the same or similar QDs in the security ink.

For example, in some embodiments of the device of FIG. 4, thetime-varying light source 1 may excite the security ink 3, therebycausing the security ink to emit an emission spectrum having first andsecond distinct regions which are characterized by first and seconddistinct lifetimes. The first optical element 7 may be disposed in afirst optical path which includes the first photodetector 6, and thesecond optical element 9 may be disposed in a second optical path whichincludes the second photodetector 8. In such a configuration, the firstoptical element 7 may act to allow only the first region of the emissionspectrum from an optical signal to pass through it, and the secondoptical element 9 may act to allow only the second region of theemission spectrum from an optical signal to pass through it. Amicrocontroller 11 typically in electrical communication with the first6 and second 8 photodetectors that may then determine thephotoluminescence lifetime of the security ink 3 over the first andsecond regions by monitoring the time or frequency response of the first6 and second 8 photodetectors. Additionally, a phase measuring device 10may determine a phase relationship between the electrical signalproducing time-varying light 2 and the electrical response of the first6 and second 8 photodetectors, and then provide that phase informationto the microcontroller 10 for determination of the first and seconddistinct lifetimes.

Additional spectral resolution may be achieved by choice of thephotodetectors. For example, a typical low-cost photodetector is asilicon photodiode which has an absorption onset of about 1100 nm. Whensuch a photodetector is combined with the QDs having the absorptionspectrum 12 shown in FIG. 5, which allow only light with wavelengthslonger than 600 nm, the resulting combination selects only emission inthe range of 600 to 1100 nm. Such a set-up would allow for detection ofthe photoluminescence 14 shown in FIG. 5, enabled by the largeseparation 13 between the absorption and emission of CuInZnSeS QDs.Typical QDs would significantly self-absorb their own PL, preventing itsdetection. Choosing a different filter and/or a different photodetectorwill adjust the spectral resolution of the detection so that specificbands of the photoluminescence (such as that shown in FIG. 6) can beselected for temporal characterization. FIG. 7 shows the PL decay from amixture of different CuInZnSeS QDs, where the PL from each type of QD isselected by a monochromator (circles and squares) having singleexponential decays of 209 ns (observed near 700 nm) and 417 ns (observednear 550 nm).

5. Making and Using the Best Mode

In the best mode of the system depicted in FIG. 4, QDs may be added toan existing ink that will typically result in a polymer matrix beingformed for an added pigment such as QDs. The ink containing the QDs maythen be applied to a substrate by any suitable method of ink depositionincluding, but not limited to, inkjet printing, stamping, scribing,spraying, or other suitable marking methods as are known to the art. Thedetector utilized in this methodology is preferably a compact andhandheld device which preferably includes a pulsed LED, color-selectivefilters, photodetectors, at least one microcontroller, and othernecessary electronics (such as, for example, a lock-in amplifier). Suchdevices are commercially available, and may be manufactured usingtechniques that are well known in the consumer electronics industry.

For the overt mode shown in FIG. 1 (described below), the security inkis illuminated by a handheld light source (such as, for example, a blueor UV LED flashlight), and the resulting visible photoluminescence isobserved visually for a simple, low-tech, first authentication, asdesired. Counterfeiters may erroneously believe that the overt modeshown in FIG. 1 is, in fact, the only security feature, and may thusfail to ensure that the covert modes shown in FIGS. 2-4 are adequatelyimparted.

The compositions, systems and methodologies disclosed herein areespecially suitable for validating the authenticity of high value items.Such validation may occur in the time-domain or the frequency domain.

In the time domain version of the system depicted in FIG. 4, the lightsource 1 is triggered to emit pulses of light 2 at multiple differentfrequencies. Thus, in the time domain, the resulting excitation (lightsource) signals are manifested as short-duration step functions.

The frequencies of excitation should be of the order of the inverse ofthe PL lifetimes of the security ink to be characterized. For example,at a very low frequency compared with the inverse of the PL lifetime,the average amount of light reaching the detectors will depend linearlyon the amplitude and the frequency of the excitation, since the ink canfully relax between pulses. At a very high frequency compared with theinverse of the PL lifetime, the average amount of light reaching thedetectors will depend on the amplitude of the excitation, but will notdepend much (if at all) on the frequency of excitation (or the PLlifetime) because the PL of the ink will only slightly decay before thenext pulse comes to re-excite the ink. Therefore, if two or morefrequencies are chosen to excite the ink in the range of the inverse ofthe lifetimes to be measured, upper and lower bounds may be placed onthe PL lifetime of the ink, thereby validating the covert feature. Usingspectral selection of the PL of the ink adds additional PL lifetimebounds for different bands of the emission spectrum, therebystrengthening the security.

The time-domain approach is simple in that only the average power fromthe photodetectors must be observed, thus simplifying the electronics.However, multiple frequencies of excitation must be used, which couldlengthen the time needed for confident authentication.

In the frequency domain version of the system depicted in FIG. 4, thelight source 1 is triggered to emit sinusoidal light 2 at a singlefrequency or at multiple frequencies. Consequently, in the frequencydomain, the signals are manifested as delta functions at the givenfrequencies. It is preferred that the frequency of excitation is on theorder of the inverse of the PL lifetimes of the security ink to becharacterized.

The electrical impulse creating the excited light is sent to a lock-inamplifier or other phase analyzer that compares it to the electricalimpulse(s) coming from the photodetector(s) at the same frequency. Thelock-in amplifier or other phase analyzer then determines the phaserelationship between the signals and the phase differences are relatedto the lifetimes of the PL detected (unknown) and the frequency of theexcitation (known). Hence, by using the phase difference between theexcitation and PL emission from the ink, the lifetime of the ink may bedetermined. Using spectral selection of the PL of the ink adds PLlifetime information for different bands of the emission spectrum,thereby strengthening the security.

The frequency-domain approach is more complicated than the time-domainapproach because it requires lock-in detection or other phase analysishardware. However, fewer (or even one) frequencies of excitation may beused in this approach, which will typically shorten the time needed forconfident authentication.

FIG. 8 illustrates a particular, non-limiting embodiment of a process inaccordance with the teachings herein in which PLs are determined for oneor more portions of an emissions spectrum. As seen therein, the processbegins with determining whether an article to be authenticated containsa security mark 15. If not, the article is not authenticated 16, and theprocess ends.

If the article does contain a security mark, then the security mark isirradiated with a time-varying light source 17. A portion of theresulting emission spectrum is then selected, and the photoluminescencelifetime (PL) is measured 18. A determination is then made as to whetherthe measured PL is within predetermined upper and lower bounds for theselected portion of the emissions spectrum 19. If not, the article isnot authenticated 16, and the process ends. If so, a determination ismade as to whether the PL has been measured over an adequate number ofportions of the emissions spectrum 20. If not, the process is passed tostep 17. If so, the article is authenticated 21, and the process ends.

FIG. 9 illustrates another particular, non-limiting embodiment of aprocess in accordance with the teachings herein in which PLs aredetermined for one or more portions of an emissions spectrum bymeasuring the phase difference between a first signal used to generatethe light used to irradiate an article, and a second signal produced bya photodetector that detects emissions from the irradiated article.

As seen therein, the process begins with determining whether an articleto be authenticated contains a security mark 115. If not, the article isnot authenticated 116, and the process ends. If the article does containa security mark, then the security mark is irradiated with atime-varying light source 122 produced by an electrical signal A. Aportion of the emission spectrum is then detected 123 with aphotodetector that produces an electrical signal B. The PL lifetime isthen determined 124 by measuring the phase difference between signals Aand B.

A determination is then made as to whether the PL lifetime is within thepredetermined upper and lower bounds for the selected portion of theemission spectrum 125. If not, the article is not authenticated 116, andthe process ends. If so, a determination is made as to whether the PLhas been measured over an adequate number of portions of the emissionsspectrum 126. If not, the process is passed to step 122. If so, thearticle is authenticated 121, and the process ends.

6. Examples

The following examples are non-limiting, and are merely intended tofurther illustrate the compositions, systems and methodologies disclosedherein.

Example 1

This example illustrates the use of overt authentication as both a quickauthentication method and a “red herring”, that is, a feature intendedto fool or frustrate counterfeiters.

The device utilized in this example is depicted schematically in FIG. 1.As seen therein, the device comprises a light source 1 (which may be,for example, a blue or UV LED flashlight) emitting an excitation 2 upona security ink containing QDs 3 applied to a substrate 4. Thephotoluminescence 5 from the ink in the irradiated substrate 4 is thenobserved and spectrally resolved by an observer's eye 6. This modeexemplifies the way that photo luminescent security inks are typicallyauthenticated, and is still an available mode for the systems andmethodologies disclosed herein. More importantly, a counterfeiterseeking to circumvent the security may believe that this mode is theonly mode of authentication, and hence this mode may serve as a “redherring” to frustrate the efforts of counterfeiters. It is possible tocreate an ink with different materials such as dyes or other types ofQDs that will appear by eye the same using this overt feature, but underthe other modes will not be authenticated.

As a test of this mode, CuInZnSeS QDs were dissolved in octane at 50mg/mL and deposited onto a paper substrate. Under blue and UV LEDflashlights, the deposited ink, which otherwise has a light yellow hue,glowed a bright orange.

Example 2

This example illustrates the use of covert authentication using asingle, un-filtered photodetector.

As seen in FIG. 2, a system is provided in which a light source 1 (suchas, for example, a blue or UV LED) emits a time-varying excitation 2upon a security ink containing QDs 3 applied to a substrate 4. Thetime-varying photoluminescence from the irradiated ink 5 is measured bya photodetectors 6. Spectral resolution is achieved by choice of thephotodetector 6.

Example 3

This example illustrates the use of covert authentication using asingle, filtered photodetector.

As seen in FIG. 3, a system is provided in which a light source 1 (whichmay be, for example, a blue or UV LED) emits a time-varying excitation 2upon a security ink containing QDs 3 applied to a substrate 4. Thetime-varying photoluminescence from the irradiated ink 5 is measured bya photodetector 6 after being spectrally resolved using a spectrumselecting component 7. In some embodiments, the spectrum selectingcomponent may comprise a thin film containing non-emissive orweakly-emissive versions of the same or similar QDs in the security ink.Additional spectral resolution is achieved by choice of thephotodetectors.

As a test of this mode, a mixture of two different CuInZnSeS QDs weredissolved in octane at 50 mg/mL and deposited onto a paper substrate.The resulting spectrum is shown in FIG. 6 (CIS QD 1 and CIS QD 2,squares). Under 445 nm excitation (blue), the PL decay was measured inthe range of from 540 to 560 nm, selecting only the emission from CIS QD2. The PL decay was measured using time-resolved single photon counting(Horiba FluoroMax 4 system) and single exponential decay of 417 ns wasobserved (see FIG. 7, circles).

7. Additional Comments

Various modifications, substitutions, combinations, and ranges ofparameters may be made or utilized in the compositions, devices andmethodologies described herein.

For example, in some embodiments, the photoluminescence of the securityink to be characterized by light emission may have wavelengths in therange of 400 nm to 1400 nm, more preferably in the range of 500 nm to1300 nm, and most preferably in the range of 550 nm to 1200 nm.

In some embodiments, the photoluminescence of the security ink may becharacterized by a lifetime of more than 100 ns, more than 150 ns, morethan 200 ns, or more than 300 ns. Preferably, however, thephotoluminescence of the security ink is less than 1 ms.

In some embodiments, the photoluminescence of the security ink may becharacterized by a lifetime that varies by at least 50 ns, by at least70 seconds, or by at least 100 ns across the emission spectrum.

In some embodiments, the photoluminescence of the security ink may becharacterized by a quantum yield of at least 30%, at least 50%, at least70%, or at least 80%.

Various light sources may be utilized in the devices and methodologiesdescribed herein to excite the security ink and/or authenticate anarticle bearing the ink. Preferably, these light sources are LED lightsources featuring one or more LEDs, and more preferably, these lightsources are selected from the group consisting of UV LEDs, blue LEDs,green LEDs and red LEDs.

The light sources utilized in the devices and methodologies describedherein may oscillate at various frequencies. Thus, for example, theselight sources may oscillate at frequencies of less than 40 MHz, lessthan 30 MHz, less than 10 MHz, or less than 5 MHz.

Various photodetectors may be utilized in the devices and methodologiesdescribed herein to analyze emissions received from an article exposedto radiation for the purposes of authentication. Thus, for example, thephotodetector may selectively absorb light with wavelengths shorter than(acting as a short pass filter) 1200 nm, shorter than 1100 nm, shorterthan 1000 nm, shorter than 900 nm, shorter than 800 nm, shorter than 700nm, or shorter than 600 nm.

Various optical elements may be utilized in the optical paths of thedevices and methodologies described herein. For example, in someembodiments, a spectrum selecting optical element may be placed in theoptical path between the irradiated article and the photodetector, andthrough which the photoluminescence passes before reaching thephotodetector. Such an optical element may include, for example, one ormore elements selected from the group consisting of light filters,quantum dot films and colored glasses. A spectrum selecting opticalelement of this type may allow only a given portion of the spectrum topass through from an optical signal incident on the spectrum selectingoptical element. By way of example, some embodiments may feature a firstspectrum selecting optical element disposed in a first optical pathbetween the irradiated article and a first photodetector, and a secondspectrum selecting optical element disposed in a second optical pathbetween the irradiated article and a second photodetector. Such anarrangement allows a microcontroller to determine the lifetime ofphotoluminescence over two distinct optical regions of the emissionspectrum. Of course, it will be appreciated that a similar approach maybe utilized to determine the lifetimes of photoluminescence over anydesired number of distinct optical regions of the emission spectrum.

In some embodiments, two or more distinct types of quantum dots may beutilized in the systems, methodologies and compositions describedherein. These quantum dots may be compositionally distinct. For example,the security inks described herein may comprise a first type of quantumdot based on a first chemistry, and a second type of quantum dot basedon a second chemistry which is distinct from the first chemistry. Thus,for example, the first type of quantum dot may comprise, for example,CuInS₂, while the second type of quantum dot may comprise AgInSe₂.Similarly, the security inks described herein may comprise a first typeof quantum dot based on a first set of dimensions (or distribution ofdimensions) of the quantum dots, and a second type of quantum dot basedon a second set of dimensions (or distribution of dimensions) of thequantum dots which is distinct from the first set of dimensions (ordistribution of dimensions) of the quantum dots. Thus, for example, thefirst type of quantum dot may comprise generally spherical quantum dotshaving a first diameter (e.g., 10 nm), and the second type of quantumdot may comprise generally spherical quantum dots having a seconddiameter (e.g., 30 nm).

Various phase analyzers may be utilized in the systems and methodologiesdescribed herein. These devices may include, but are not limited to,lock-in amplifiers, impedance gain phase analyzers, oscilloscopes, andnetwork analyzers. Typically, such devices operate by measuring a phaserelationship between a time-varying excitation and a time-varyingphotoluminescence for a security ink of the type disclosed herein.

The above description of the present invention is illustrative, and isnot intended to be limiting. It will thus be appreciated that variousadditions, substitutions and modifications may be made to the abovedescribed embodiments without departing from the scope of the presentinvention. Accordingly, the scope of the present invention should beconstrued in reference to the appended claims.

Moreover, it is specifically contemplated that the features described inthe appended claims may be arranged in different combinations orsub-combinations without departing from the scope of the presentdisclosure. For example, it is contemplated that features set forth intwo or more claims may be combined into a single claim without departingfrom the scope of the present disclosure, whether or not the resultingcombination of features is explicitly disclosed elsewhere in theappended claims or disclosure.

What is claimed is:
 1. A method for authenticating an article, whereinauthenticating the article includes determining that the article belongsto a set of authentic articles, wherein each member of the set ofauthentic articles bears an inked security mark that emits aphotoluminescence emission spectrum in response to being excited by atime-varying light source that emits pulses of light at a plurality offrequencies, and wherein the photoluminescence emission spectrum ischaracterized by first and second distinct regions which arecharacterized by respective first and second distinct lifetimes, themethod comprising: determining whether the article to be authenticatedcontains a security mark having the first and second distinct regions;if the article contains a security mark having the first and seconddistinct regions, irradiating the security mark with an instance of saidtime-varying light source, and determining the upper and lower boundsfor the lifetime of at least one portion of a photoluminescence spectrumemitted by the irradiated security mark; and authenticating the articleonly if (a) the article contains a security mark having said first andsecond distinct regions, and (b) the irradiated security mark exhibits aphotoluminescence in said first and second distinct regionscharacterized by said first and second lifetimes; wherein the securitymark comprises a first set of quantum dots in the first region, and asecond set of quantum dots in the second region.
 2. The method of claim1, further comprising: using at least one spectrum selecting opticalcomponent to choose each portion of the photoluminescence spectrum forwhich the photoluminescence lifetime of said security mark isascertained, wherein said at least one spectrum selecting opticalcomponent is selected from the group consisting of light filters,quantum dot films and colored glasses.
 3. The method of claim 1, furthercomprising: using the average light intensity collected from said atleast one photodetector when said security mark is irradiated with saidplurality of frequencies to compute upper and lower bounds for saidfirst and second lifetimes.
 4. The method of claim 1, wherein the firstand second sets of quantum dots are chemically or dimensionallydistinct.
 5. The method of claim 1, wherein the security mark comprisesquantum dots with compositions selected from the group consisting ofCuInS₂, CuInSe₂, AgInSe₂, AgInS₂, ZnSe and ZnS.
 6. The method of claim1, wherein the photoluminescence emission spectrum of the security markis characterized by light emission having wavelengths in the range of450 nm to 1250 nm.
 7. The method of claim 1, wherein thephotoluminescence of the security mark is characterized by a lifetime ofmore than 40 nanoseconds but less than 1 millisecond.
 8. The method ofclaim 1, wherein the photoluminescence of the security mark ischaracterized by a lifetime that varies by at least 5% across saidphotoluminescence emission spectrum.
 9. The method of claim 1, whereinthe photoluminescence of the security mark is characterized by alifetime that varies by at least 50 ns across said emission spectrum.10. The method of claim 1, wherein the photoluminescence of the securitymark is characterized by a quantum yield that is greater than 50%. 11.The method of claim 1, further comprising a handheld device thatincludes said light source and at least one photodetector.
 12. Themethod of claim 11, further comprising at least one spectrum-selectingcomponent through which said photoluminescence must pass before reachingsaid at least one photodetector.
 13. The method of claim 12, furthercomprising: determining said photoluminescence lifetime based on thetime-varying average light intensity measured by said at least onephotodetector.
 14. The method of claim 1, further comprising determiningsaid photoluminescence lifetime based on the phase difference betweensaid light source and said photoluminescence using a phase analyzer. 15.The method of claim 1, wherein the security mark comprises quantum dotshaving an inner core and an outer shell.
 16. A method for authenticatingan article as belonging to a set of authentic articles, wherein eachmember of the set of authentic articles bears an inked security markthat emits a photoluminescence spectrum in response to being excited bya time-varying light source, and wherein the emitted photoluminescencespectrum is characterized by a range of lifetimes, the methodcomprising: determining whether the article to be authenticated containsa security mark; if the article contains a security mark, (a)irradiating the security mark with a time-varying light source, whereinsaid time-varying light is created with a first electrical signal, and(b) capturing a portion of the emission spectrum of the irradiatedarticle with at least one photodetector; determining the phasedifferences between the first electrical signal and a second electricalsignal of the same frequency which is received from said photodetectorin response to the captured portion of the emission spectrum;determining the lifetimes of the photoluminescence of the irradiatedsecurity mark from the determined phase differences; and authenticatingthe article only if (a) the article contains a security mark, and (b)the irradiated security mark emits a photoluminescence spectrum whosedetermined lifetimes fall within the range of lifetimes characteristicof an authentic article; wherein said security mark emits an emissionspectrum having first and second distinct regions which arecharacterized by first and second distinct lifetimes; wherein theauthenticity of the article is verified only if the security markexhibits a photoluminescence which has first and second lifetimes thatfall within first and second ranges of appropriate values for the firstand second regions, respectively; and wherein the security markcomprises a first set of quantum dots which exhibit photoluminescenceover the first region, and a second set of quantum dots which exhibitphotoluminescence over the second region.
 17. The method of claim 16,wherein the security mark comprises quantum dots with compositionsselected from the group consisting of CuInS₂, CuInSe₂, AgInSe₂, AgInS₂,ZnSe and ZnS.
 18. The method of claim 16, wherein determining the phasedifferences between the first and second electrical signals includes:inputting the first and second electrical signals into a lock-inamplifier; receiving the phase relationship between the first and secondsignals from the lock-in amplifier; and determining the phasedifferences between the first and second electrical signals from thephase relationship.
 19. The method of claim 16, further comprising:using at least one spectrum selecting optical component to choose eachportion of the photoluminescence spectrum for which thephotoluminescence lifetime of said security mark is ascertained, whereinsaid at least one spectrum selecting optical component is selected fromthe group consisting of light filters, quantum dot films and coloredglasses.
 20. The method of claim 16, further comprising: using theaverage light intensity collected from said at least one photodetectorwhen said security mark is irradiated with said plurality of frequenciesto compute upper and lower bounds for said first and second lifetimes.21. The method of claim 16, wherein the first and second sets of quantumdots are chemically or dimensionally distinct.
 22. The method of claim16, wherein the photoluminescence emission spectrum of the security markis characterized by light emission having wavelengths in the range of450 nm to 1250 nm.
 23. The method of claim 16, wherein thephotoluminescence of the security mark is characterized by a lifetime ofmore than 40 nanoseconds but less than 1 millisecond.
 24. A method forauthenticating an article, wherein authenticating the article includesdetermining that the article belongs to a set of authentic articles,wherein each member of the set of authentic articles bears an inkedsecurity mark that emits a photoluminescence emission spectrum inresponse to being excited by a time-varying light source that emitspulses of light at a plurality of frequencies, and wherein thephotoluminescence emission spectrum is characterized by first and seconddistinct regions which are characterized by respective first and seconddistinct lifetimes, the method comprising: determining whether thearticle to be authenticated contains a security mark having the firstand second distinct regions; if the article contains a security markhaving the first and second distinct regions, irradiating the securitymark with an instance of said time-varying light source, and determiningthe upper and lower bounds for the lifetime of at least one portion of aphotoluminescence spectrum emitted by the irradiated security mark; andauthenticating the article only if (a) the article contains a securitymark having said first and second distinct regions, and (b) theirradiated security mark exhibits a photoluminescence in said first andsecond distinct regions characterized by said first and secondlifetimes; wherein the security mark comprises a set of quantum dots inboth the first and second region.